I have been reading the PhD dissertation of Vaclav Dostal for information and inspiration, and I came across a graphic which shows the difference between the bulk of a steam turbine system, a helium turbine (proposed for high-temperature nuclear reactors) and a CO2 turbine system (a cheaper alternative to the helium turbine). Here it is:

In raw numbers, the CO2 power turbine is 0.6 meters radius (about 4 feet diameter) and 55 centimeters (less than 2 feet) long. The compressors would be even smaller. Yet this small bundle of turbomachinery, which would easily fit in a couple of pickup trucks, could crank out 450 megawatts.

Using the supercritical CO2 recompression cycle, the turbomachinery for a gigawatt powerplant could fit in your bedroom. Think about that for a minute.

Small is not just beautiful. Small is also cheaper, easier to build and quicker to install. In the necessary repowering of the energy systems of the USA and the world, small, beautiful, efficient, elegant systems like this need to be pushed to center stage.

These things are amazing. Less complex, and smaller is cheaper, and CO2 is easier to handle than helium (bigger molecules).

There are also some concerns about helium shortages, a bit of irony considering only hydrogen is more abundant in the universe. By contrast, CO2 has negative future value!

It might be sustainable to distill helium from the air. I wonder what the energy requirement for that would be? Probably huge, given it's low percentage in the atmosphere, and lowest boiling point of all the atmospheric gasses.

On the downside, CO2 cycles are less well developed right now, and CO2 is mildly corrosive on the equipment. Not major issues I'd say. Overall, CO2 looks like the best choice for a non combustion Brayton cycle. What is the temperature limit (max) that CO2 cycles can operate at?

The really interesting thing is the efficiency. If I'm reading Dostal's paper correctly, the supercritical CO2 turbine is more efficient at 550°C than the helium turbine. Dostal's paper barely touches corrosion, and doesn't have anything on corrosion from CO2 that I can recall seeing. (The PDF is scanned page images, not OCR'ed text, so it cannot be searched. Very annoying.)

I came into this looking at CO2 turbines as a way to convert coal plants to fully-sequestered systems. GE has gas turbines operating at up to 1360°C turbine inlet temperature, which would boost the efficiency of a supercritical CO2 system sky-high. It would not be overly difficult to hit this temperature with combustion of CO (from a coal gasifier) and O2, and the product would be a sequestration-ready CO2 stream.

It looks like basically a tradeoff between lower temperature and higher pressure. Given the advantages for lower temperature power plant operation (especially for nuclear and solar thermal powerplants) this looks like a good tradeoff.

Can you really take a SCO2 cycle to 1300 degrees celcius? Dostal is talking about 700 degrees celcius for an advanced design.

Corrosion might be a problem with some materials particularly carbon steel, but a very compact Brayton would not use much material so exotic expensive alloys, cermets, and ceramics could be used that are more resistant to corrosion. Dostal talks about stainless steel and titanium.

The power density that the materials will have to endure is massive, perhaps on the order of a MegaWatt per liter!

With coal you'd have all sorts of problems with contaminants that you don't want in your cycle, right? You'd need a very selective filtering and washing system or something like that. Heavy metals, sulphur, mineral ash... don't want that in your Brayton!

I have no idea if we currently have materials capable of operating at both the temperatures used in cutting-edge aeroderivative gas turbines and the pressures and stress levels implied by the fluid density of CO2 at 20 MPa. I'd guess that we do, though the size of the turbine might be limited by mechanical stress.

One other factoid that I find interesting: a powerplant using an oxygen-blown coal gasifier and producing liquid CO2 would be able to use liquid CO2 instead of water to produce the coal slurry. This reduces the heat of vaporization of the slurry and increases the cold-gas efficiency of the gasifier.

There is no question that these represent a great alternative for both nuclear, and (coal)thermal plants, although the authors are focused on nuclear.

A later paper, by the same author, has some very good comparisons of cycle alternatives with SCO2http://stc.fs.cvut.cz/pdf/KulhanekMartin-319574.pdf

It sounds like the limit of 550C for CO2 is some reactor specific reason. In the 1968 paper, Angelino used 700C is the turbine inlet temperature.

Agree that this would appear to be an excellent substitute for the steam cycle for a coal plant. The supercritical steam plants are of similar efficiency, but way more expensive.

The power density is 395MW per sq.m of turbine area - a 4" turbine would develop 2.9MW!.

There is actually equipment out there today that comes close to this sort of power density, though not at hundreds of megawatts. Hydraulic pumps and motors run at these pressures (and much higher) and have a this sort of power density. They don't have to handle these temperatures though.

I find the CO2 coal slurry an interesting idea, though for a different reason. There is ongoing research for using SCO2 for oilsands extraction, instead of the current steam method. SCO is very good at oil extraction from anything, it is used for drycleaning, coffee decaffeination and plant essential oil extraction (eucalyptus, etc). One of the things they find after doing the extraction on oilsands is that a good portion of the heavy bitumen gets upgraded (hydrocracked, I guess) and comes out as a light, sweet crude! if you did SCO on bituminous coal, you may well get a light oil product, and then a carbon residue, which can then be gasified and burnt. At current coal and crude prices, if the oil fraction turned out to be 6% by weight of the coal, its value, as crude, would be equal to the 94% of the remaining coal! For the high sulphur Wyoming coal, it would triple the value of the coal. It's possible that here might be a coal to liquid process that actually makes economic sense. It would also make coal fired powerplants so profitable that none would ever get shut down unless the CO2 price is through the roof.If, and it's a big if, this could be extrapolated to all 1 billion tons of US coal, 5% as extracted oil would be 1 million barrels a day.To make it work you would have to take the CO2/coal slurry below the critical pressure and condense out the oil, but you would also be able to separate out a lot of the ash too, then take the coal/CO2 critical again, blow in your O2 and carry on.As long as the turbomachinery could handle the impurities, then all these would get sequestered too! The only stuff that would leave the closed cycle is the oil extract, and the ash, both of which are easily controlled and the ash can be returned to the old minespace.

I think the most interesting thing is that it would also open the way to downsize new coal plants, you wouldn't need to go to 500MW for economies of scale like you do with steam. When a 50 or 100MW system can fit on a semi trailer or three, it may become more economic to start taking the power plants back to the coal mines, or, with full sequestration, closer to the point of use. In any case, lots of possibilities open up with these systems.

I wonder if the US Navy ever looked at these for nuclear ships and subs? The space savings would be worth their volume in gold...

That paper only lists cycles with a high-side temp of 550°C (doesn't do much for my thermodynamics bleg).

I'd wager that the light-oil extraction from tar sands is from fractionation, not cracking. CO2 is relatively inert, the temperatures involved are low, and it has no hydrogen to upgrade heavy hydrocarbons. The solvent effect may help by fractionating coal (esp. separating water which reduces the lower heating value), but it's not going to turn it into oil.

On second thought, gasifying with CO2 will cut the amount of hydrogen in the syngas. This will both increase the amount of CO to be extracted (which has energy overhead) and reduce the amount of hydrogen for carbon-free fuel and chemical feedstock. It may not be the optimal way to go.

Hadn't thought that the oilsand result could be fractionation, but even that is a potential improvement over the current steam method. Having said that, this is still stuck in the university research lab and none of the oilsands players are funding it (or anything else, at the moment).

I was amazed to find there is an off the shelf residential CO2 heat pump being sold in Japan, though not sure if it is a supercritical cycle.

And almost every plant processing industry has experimented with it too.

The main thing I take away from all this information is that there are many, and increasing, industrial uses for SCO2.

As for power generation it looks good, great, even. I find the prospect of smaller scale (1-50MW) SCO2 cycle power plants fascinating. It would open up a lot of options for MSW waste to energy without needing complex gasification control and filtration systems.

While it may be a good substitute for large scale helium cycle, I think it looks like a great substitute for small-medium scale steam , an area where steam is at its least cost effective point, but presently there is no practical alternative. If this comes about, the commercial shipping industry might take a look too